The invention is directed to improvements in detectors which sample the peak magnitude of an electrical input signal and store the sampled magnitude for further use. Such detectors are usually referred to as "sample and hold" or "peak" detectors.
One of the simplest forms of peak detectors includes a diode for receiving the electrical input signal and for driving a capacitor which stores the peak value of the input signal. Variations of such schemes may include replacing the diode with a transistor which may be turned off and on at appropriate intervals for sampling a selected portion of the input signal.
The above-mentioned types of detectors suffer from several problems, one of which is the inability of the capacitor to store a voltage which is nearly exactly equal in amplitude to the peak voltage associated with the electrical input signal. For example, where the peak detector includes the transistor-capacitor combination, the voltage stored on the capacitor is invariably lower than the voltage of the input signal which is applied to the base of the transistor because of the transistor's base-emitter voltage drop. In some applications, the base-emitter voltage difference between the capacitor voltage and the voltage associated with the sampled signal is significant and undesirable.
The base-emitter voltage difference referred to above also gives rise to another problem due to the fact that, as the temperature of the transistor changes, its base-emitter voltage also changes. Hence, the capacitor voltage also varies as a function of temperature even though the voltage associated with the input signal may remain constant. Accordingly, the sampling voltage on the capacitor does not track with the input signal as temperature changes.
The conventional peak detector described above also suffers from the fact that its stored voltage varies with sampling time. This result occurs because the forward resistance of the base-emitter junction of the transistor (or diode) used as a peak detector varies as a function of the voltage across that junction. Therefore, as the capacitor charges and its voltage approaches the peak voltage of input signal, the forward resistance of the junction increases, thereby reducing the capacitor's rate of charge. Hence, the voltage on the capacitor only approaches, but never reaches, the peak amplitude of the input signal. The stored voltage on the capacitor is, therefore, a function of the sampling time. For applications where a very short sampling time is necessary, the conventional peak detector is very inefficient and inaccurate.
Yet another problem which arises with conventional detectors is their susceptibility to noise. Specifically, if the input signal includes random noise the voltage on the capacitor may rise to the peak value of the noise. Clearly, it is preferable for the capacitor voltage to be less sensitive to such noise.
For reasons set forth above, prior detectors have been less than satisfactory for a variety of applications, particularly applications in which the capacitor voltage is required to very accurately correspond to and track with the voltage associated with the sampled signal.